Wavelength-selective negative photoresist for photolithography suitable for generating microstructures with up to three distinct height levels

Abstract

Grayscale lithography is an important process for structuring polymeric materials for many applications in microsystem engineering. However, until now grayscale lithography is mostly used for patterning structures of the same height and remains largely unsuitable for generating more complex surfaces. Here, we present a novel method which enhances the accessible height ranges and surface topographies from one to a total of three discrete structure heights which can be accessed independently. This allows generating microstructures for, e.g., microfluidics and microoptics, with significantly enhanced structural complexity. The method does not require complex preprocessing steps. It is based on the varying absorption parameters of wavelength-selective absorbers mixed into a photocurable material. While the actual photoinitiator used covers all spectra, the absorbers are wavelength-dependent. These absorbers are chosen such that they absorb light in a narrow wavelengths regime while affecting the other processing wavelengths significantly less. We designed a photoresist with three distinct wavelengths: 365 nm, 415 nm, and 440 nm using polyethylene glycol diacrylate (PEGDA) as main resin material. Microfluidic channel structures were fabricated by transferring the generated structure into polydimethylsiloxane (PDMS).